The owner of an ebike fitted with I assume the first iteration of the nuvinci hub,

commented that he gets “about 47km/h on flat ground with the NuVinci wheel and 51km/h with a normal wheel. The transmission steals quite a bit of power from the motor.”

That appears to be roughly 92% Efficiency in this particular usage case, although I presume it must be worse at lower speeds.

I just want to be able to say, before I plunk down $3k, “Alright, the effort that would normally having me going 10mph will now have me going 20mph and the charge would still last 50 miles, which is more than enough to get me back and forth to work without being all sweaty. Sold!” Right now it’s mostly, “Meh, I’ll just split the difference and go 15mph on my lighter current setup.”

On the other hand, NuVinci is a sort of Traction Drive. Traction drive is usually unfamiliar even to mechanical engineers and mostly regarded as a magic to not-engineers. In a traction drive, flat-surfaced rollers contact with other flat-surfaced rollers without teeth. And amazingly the rollers transfer power without slippage.

The secret of this magic is from the lubricant called ‘traction oil’. Traction oil dramatically increases friction at contact-surfaces of rollers : Imagine a bicycle having two steel wheels but no rubber tires on them. The bicycle is running on a large steel plate. Yes, there will be slippages whenever high pedaling torque is given. But if traction oil is covered on the steel plate then No Slip.

What I hope to note is ‘operating conditions’ of the drive. Transferring high torque without slippage needs high friction and it’s possible by increasing contact pressure between rollers. High friction causes high energy loss. So a traction drive is known to be suitable for relatively low torque & high speed conditions. ※ All experimental studies on traction drives what I saw described their input speed is over 1000rpm(mostly around 1500rpm).

The NuVinci maker will not reveal the efficiency. ※ I guess the minimum efficiency could be under 85%. Most people can’t feel such amount of energy loss on rides because the rolling resistances of tires are bigger.

Anyway it doesn’t mean NuVinci is useless. NuVinci is a surely good invention providing differentiated characteristics. I think the drive is effective but (just) not efficient.

That makes me all the more curious about the NuVici Harmony though, as I imagine that this problem disappears with CVT. I can see the appeal of e-bikes for some… if you look at the number of old Dutch couples taking tours on the countryside riding e-bikes, you can imagine that there’s a market for CVT-autoshift-pedelec machines. I could probably sell my fairly sedentary parents on those, too.

I will look more seriously at these things when some company steps away from the marketing and crunches the numbers to show a real benefit. I’m not going to double the weight of my bike and quintuple the price just to get a small electric boost without having to shift gears. I’d rather drop that money on something I didn’t have to pedal at all, which is already what most people do.